Method of frequency-stabilization of a single-made gas laser

ABSTRACT

The method consists in applying a steady axial magnetic field to the active gaseous medium, in determining either the value of said steady magnetic field which corresponds to the laser-cavity oscillation frequency to be stabilized or the length of the laser cavity corresponding to a set value of the steady magnetic field so that the oscillation frequency should correspond to the peak value of one of the two Doppler curves obtained as a result of the Zeeman effect by splitting the Doppler curve which corresponds to a zero value of the steady magnetic field, in superimposing an alternating axial magnetic field on said steady magnetic field and causing the split Doppler curve to oscillate about a position corresponding to the selected value of the steady magnetic field in order to modulate the output light intensity of said laser, in detecting the modulated output intensity, in comparing the variations in modulation of said output intensity with those of the alternating magnetic field by means of a detector which delivers a signal so that the lightemission frequency of the laser may be corrected by said signal and by a change in length of the laser cavity.

United States. Patent Le Floch Mar. 14, 1972 [54] METHOD OF FREQUENCY-STABILIZATION OF A SINGLE-MADE GAS LASER [72] Inventor: Albert Le Floch,7, square du Bois Perrin,

' 35 Rennes, France [22 Filed: Dec. 4, 1970 211 Appl.No.: 95,283

[30] Foreign Application Priority Data Dec. 12, 1969 France ..6943193[52] US. Cl ..33l/94.5 [51] Int. Cl ..H0ls 3/10 [58] Field ofSearch..33l/94.5;350/l60 [56] References Cited UNITED STATES PATENTS 3,534,29210/1970 Cutler ..331/94.5

Primary Examiner-William L. Sikes Attorney-Cameron, Kerkam & SuttonM/RROR PHOTOELECTR/ OETfOTOR C DIRECT CURRENT PHASE ABSTRACT The methodconsists in applying a steady axial magnetic field to the active gaseousmedium, in determining either the value of said steady magnetic fieldwhich corresponds to the lasercavity oscillation frequency to bestabilized or the length of the laser cavity corresponding to a setvalue of the steady magnetic field so that the oscillation frequencyshould correspond to the peak value of one of the two Doppler curvesobtained as a result of the Zeeman effect by splitting the Doppler curvewhich corresponds to a zero value of the steady magnetic field, insuperimposing an alternating axial magnetic field on said steadymagnetic field and causing the split Doppler curve to oscillate about aposition corresponding to the selected value of the steady magneticfield in order to modulate the output light intensity of said laser, indetecting the modulated output intensity, in comparing the variations inmodulation of said output intensity with those of the alternatingmagnetic field by means of a detector which delivers a signal so thatthe light-emission frequency of the laser may be corrected by saidsignal and by a change in length of the laser cavity.

7 Claims, 6 Drawing Figures MIRROR 4 6 l3 D/RECT OURRE/V 7' GENERATOR llAMPLIFIER PATENTEUMARMIQYZ 3,649,930

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METHOD OF F REQUENCY-STABILIZATION OF A SINGLE-MADE GAS LASER Thisinvention relates to a method for stabilizing the optical frequency ofthe output light emission of a single-mode gas laser and to a gas laserfor carrying out said method.

Many laser applications such as metrology or the study of Brillouin andRaman scattering call for a vary stable optical frequency. A number ofmethods have been proposed for the stabilization of gas lasers and canbe placed in three main categories.

In accordance with the first method, two lasers are employed and oneserves to stabilize the otherv The obvious disadvantage of this methodarises from the fact that the use of two lasers entails a very highcapital outlay. The second method involves the use of a substance whichis usually a gas contained in a tank and which exhibits saturableabsorption to the laser radiation to be stabilized. The tank is placedeither inside or outside the laser cavity. This method of stabilizationis highly efficient but nevertheless suffers from a major drawback inthat a tank is introduced on the path of the light beam. Apart from theresultant increase in overall size of the device, this is liable to giverise to undesirable reflections from the tank walls and to furtherdifficulties of alignment. In the third method, a correction signalderived from the laser itselfis employed in order to stabilize theoptical frequency of the output intensity. This method provides theclearest illustration of the prior art and will be explained in detailafter reference has been made to the accompanying figures.

FIGS. 1 and 2 represent respectively the Doppler curve of the gain ofthe active medium and the output intensity as a function ofthe frequencyin a zero magnetic field.

FIG. 3 represents the Doppler curves obtained by causing a continuousaxial magnetic field to act on the active laser medium (the diagramcorresponds to a transition J=l J=; the splitting in the case ofa lineJ=l J=2 is equivalent as a first approximation if the Lande g-factorsare closely related).

FIG. 4 is a curve showing the variations in the output intensity I ofthe laser as a function of the value of the axial steady magnetic fieldH which acts on the active laser medium in respect ofa constantoscillation frequency of the cavity FIGv 5 represents the intensity S ofthe correction signal as a function of the value of the axialalternating magnetic field H, which is superimposed on the constant orsteady field in respect of three resonant frequencies of the lasercavity (curves 1, 2, 3)v

FIG. 6 represents an advantageous embodiment of the invention.

In accordance with the above-mentioned third method of the prior art,there is employed a phenomenon which is known by the name of I-Ioleburning as introduced by Bennett in I962 (W. R. Bennett, Phys. Rev.,I26, 1962, p. 580). The gas atoms which constitute the active lasermedium are endowed with motion resulting from thermal agitation. If thegain G of the light ray on which the laser emission is produced isrepresented as a function of its optical frequency, a bellshaped curve(Gaussian curve) as shown in FIG. 1 is usually obtained. This curve isknown as the Doppler curve or gain since it results from a Dopplereffect arising from thermal agitation of the gas atoms. A standing-wavestate is established within a laser cavity which is a resonant cavity,the length of said cavity being equal to a whole number of times thehalfperiod of the standing waves. It is known that a standing wave canbe split-up into two waves which have identical intensities andpropagate at equal velocities but in opposite directions. Since thelaser under consideration is of the single-mode type, said laseroscillates at only one frequency. The wave which propagates in onedirection interacts with the atoms of the active medium which have avelocity +v, for example, whereas the wave which propagates in the otherdirection interacts with atoms having a velocity v. These two wavestherefore interact with atoms which have the same velocity but are ofopposite direction. Population inversion of the energy levels of atomshaving the velocities :v decreases with respect to that of the otherenergy levels of atoms having velocities other than v. In fact, sincethe laser is of the single-mode type, the energy extracted from theactive medium is taken solely from the atoms having velocities iv whichcorrespond to the laser emission frequency. The output intensity I ofthe laser which emits at an optical frequency v is correspondinglyhigher as the gain (or population inversion) in the active medium ishigher at said frequency v. In FIG. 1, assuming that the laser operatesat the frequency 11,, a hollow portion will therefore be formed in theDoppler curve in respect of the frequency 2/, since the populationinversion and therefore the gain decreases at this frequency. The lightenergy having a frequency v (maximum value of the Doppler curve) isemitted by active gas atoms having zero velocity. That portion of theDoppler curve which is located at frequencies lower than v correspondsto atoms having negative velocities (relative to the wave) whereas theother portion of the curve which is located at frequencies higher than11., corresponds to atoms having positive velocities. Since there is astanding wave within the laser cavity, that is to say two waves havingthe same frequency and equal intensities but opposite velocities and ifone wave interacts with atoms having a velocity +v, the other wave willinteract with atoms having a velocity v. In the Doppler curve of FIG. 1,there will therefore also be a hole corresponding to an emissionfrequency 1 which is symmetrical with 1 relative to n It is thisphenomenon of appearance of two symmetrical holes in the Doppler curvewhich has been referred to as Hole burning. Since this phenomenon takesplace at all oscillation frequencies of the laser cavity and thereforeat each point of the Doppler curve, said holes cannot be shownexperimentally when the curve of output intensity I of the laser isplotted as a function of its frequency except in one case. In fact,assuming that the resonant frequency of the laser cavity is modified bychanging the cavity length, for example, and that the cavity oscillatesat the frequency 1 corresponding to atoms of zero velocity, the twowaves of opposite velocities which propagate within the laser cavitywill therefore interact with the same atoms having zero velocity. Inconsequence, the two holes of the Doppler curve which were initiallyformed in respect of the frequencies :1 and 11 will now meet and beplaced at the frequency 1 instead of having a local formation of asingle hole in the Doppler curve, there will therefore be formed twoholes. Accordingly, it is apparent that the gain of the laser in respectof the frequency 11 will have a minimum value inasmuch as the two waveswhich propagate within the laser cavity at equal but opposite velocitiesinteract with the same group of atoms having zero velocity. It is thishollow portion at the peak value of the Doppler curve which has beenrepresented in FIG. 2. The theory of this phenomenon has been given byLamb (W. E. Lamb, Phys. Rev., 134, No. 6 A, 1964, p. 1,429) and thishole in the Doppler curve at the frequency v which is observedexperimentally has been referred-to as the Lamb dip. In the Lamb theory,the variation dE/dt in time of the electric field E which is associatedwith the light wave emerging from the laser (the output intensity Iofthe laser being equal to E can be written:

dE/dt a EB E In this expression, (1 represents the gain withoutsaturation and B represents a saturation term. It is understoodintuitively that the presence of the Lamb dip is solely due to thesaturation term 8 since it can be stated in a simplified manner that thegaseous active medium is not capable of supplying a sufficient amount ofenergy at the frequency 11 the number of gas atoms which have zerovelocity being virtually insufficient.

The presence of the Lamb dip is turned to useful account for frequencystabilization of single-mode gas lasers in devices of the prior artwhich are the most closely related to the present invention. The methodconsists first in choosing as operating point of the laser the frequency11 which corresponds to the minimum value of the Lamb dip. This isachieved very precisely by judiciously selecting the value of the lengthof the laser cavity. Thereupon, one of the mirrors constituting thelaser cavity is caused to vibrate to a very slight extent, this beingtantamount to an oscillation of the optical frequency about n In otherwords, the Lamb dip is scanned about the minimum value 11 The outputintensity of the laser is modulated as a result of the periodicvariation in the resonant frequency of the cavity as produced by thevibration of one of the two mirrors. The characteristics (phase andamplitude) of said modulation are dependent on the deviation of themean" operating point of the laser with respect to the frequency v Thevariations of said modulation are then compared with those of the lengthof the cavity with the aid of adequate means which are capable ofdelivering a signal for the correction of intensity which becomesgreater as the frequency drift increases. This correction signal whichis applied to suitable means for varying the length of the laser cavitymakes it possible to correct said frequency drift. This method ofeffective stabilization is employed for frequency stabilization ofcommercially available single-mode gas lasers. However, the method has amajor disadvantage which arises from its underlying principle. In fact,the correction signal is obtained by causing a slight oscillation in theresonant frequency of the laser cavity in order to scan the Lamb dip. Inorder to stabilize the optical frequency of the laser, it is thereforefound necessary to modulate said frequency with a view to obtaining acorrection signal. In practice, steps are taken to ensure that theoscillation of the optical frequency which is chosen so as to have thelowest possible frequency is compatible with the value of the ratio ofcorrection signal to noise. Since the intensity of the correction signalis higher as the Lamb dip scanning amplitude is of greater value, saidintensity is therefore limited.

The invention provides a method and a device which meet practicalrequirements more effectively than those ofthe prior art, especiallyinsofar as the above-mentioned disadvantages no longer arise. In orderto correct any possible variations in optical frequency of the lightemission of the laser, the invention is primarily intended to obtain acorrection signal resulting from variations in output intensity of thelaser without resorting to oscillation of the length of the lasercavity. A further object of the invention is to obtain a high-amplitudecorrection signal without modifying the resonant frequency of the lasercavity.

To this end, the invention proposes a method of frequency stabilizationof a single-mode gas laser, wherein use is made of a correction signalresulting from variations in output intensity ofsaid laser and whereinsaid method consists successively:

in applying a steady axial magnetic field to the gaseous active medium,

in determining the value of said steady magnetic field which correspondsto the laser-cavity oscillation frequency to be stabilized and thereforeto a preestablished length of the laser cavity or conversely indetermining the length of the laser cavity which corresponds to apreestablished value of said steady magnetic field so that theoscillation frequency of the laser cavity should correspond to a peakvalue of one of the two Doppler curves which are obtained as a result ofthe Zeeman effect by splitting the Doppler curve corresponding to a zerovalue of the steady magnetic field, that is to say in satisfying theconditions of resonance of the magnetic Lamb dip,

in applying an alternating axial magnetic field which is superimposed onsaid steady magnetic field so that the split Doppler curve shouldoscillate about its position corresponding to the value of said steadymagnetic field, the output light intensity ofsaid laser being thenmodulated,

in detecting said modulated output intensity,

in comparing the variations in modulation of said output intensity withthose of said alternating magnetic field by means ofa detector whichdelivers a signal for correction of intensity which increases with themagnitude of the difference between said variations and,

in correcting the frequency of the light emission ofsaid laser ifnecessary by means of said correction signal and by modifying the lengthof said laser cavity.

The invention also proposes a frequency-stabilized singlemode laser forcarrying out said method and comprising a gaseous active mediumcontained in an envelope and located between two mirrors forming aFabry-Perot resonant cavity, means for exciting said gaseous medium soas to produce a population inversion, means for producing within saidactive medium an adjustable steady axial magnetic field and an adjustable alternating axial magnetic field, a detector for detecting thelight intensity emitted by said laser, means for comparing thevariations in said light intensity with the variations in saidalternating magnetic field, said comparison means being such as todeliver a correction signal, and means for varying the length of saidlaser cavity which are controlled by said correction signal.

A clearer understanding of the invention will be obtained from thefollowing description of one mode of execution of the invention which isgiven by way of example without any limitation being implied, referencebeing made to FIGS. 3 to 6 of the accompanying drawings.

When a steady magnetic field is applied to the medium, it is found that,in a direction of observation of the medium which corresponds to thedirection of the steady magnetic field, the energy levels of the atomsof said medium are split into two j+l sub-levels.

This is the Zeeman effect. Starting from a given energy level (J=l thereare therefore obtained three Zeeman sub-levels. The difference Au,between two Zeeman sub-levels is given by the relation z=g L H in whichg represents a number which is referred to as the splitting factor orLande g-factor and which has a different value in respect of each levelconsidered, {3 is the value of the Bohr magneton and H represents theintensity of the steady magnetic field which is applied to the medium.In the case of a predetermined energy level, the difference between twoZeeman sub-levels is therefore proportional to the value of the appliedmagnetic field. The Doppler curve which represents the gain as afunction of the emission frequency 21 will therefore be split into twocurves 0' and 0* each corresponding to one Zeeman sub-level. These twoDoppler curves which are shown in FIG. 3 are symmetrical with respect tothe initial frequency v of the laser emission. Let it now be assumedthat a steady axial magnetic field produces action on the laser activemedium and that the resonant frequency v, of the laser cavity isdisplaced uniformly by progressively varying the length of the lasercavity; and if 8 represents the deviation of the resonant frequency v,of the cavity with respect to the central frequency v,,(5=v,-v aresonance effect is observed in agreement with the conventional Lambtheory when we have the equality:

When this condition is satisfied, there is accordingly observed asuperimposition of the two holes in the split Doppler curve, for examplein the case of the component 0* in FIG. 3 and consequently a hole in theoutput intensity of the laser which corresponds to the conventional Lambdip of FIG. 2. It is evidently possible to set a value 8 and to observea hole in the output intensity in the case of the same condition 8=Av,,not by varying 5 but by varying the steady magnetic field. By analogy,said hole in the curve of the output intensity in the presence of amagnetic field is referred to as a magnetic Lamb dip".

Experimentally, the magnetic Lamb dip can be clearly shown by plottingthe curve representing the output intensity l of the laser as a functionof the value of the steady magnetic field H applied to the active lasermedium in respect of a given value offi (approximately I50 mc./sec. inthe case of FIG. 4). On this curve, the magnetic Lamb dip appears in theform of a hollow portion as indicated by the point K in FIG. 4. It isreadily understood that this curve in fact corresponds to a singlemodelaser and to a constant length of the laser cavity.

The method for stabilizing the optical frequency of a singlemode gaslaser consists in accordance with the present invention in applying in afirst step a steady axial magnetic field to the active medium in orderto split the initial Doppler curve into two components a and The valueof the steady magnetic field and therefore Av, is established.Thereupon, by progressively varying the oscillation frequency v, of thelaser cavity, that is to say in practice by progressively modifying itslength, the condition of observation of the resonance of the magneticLamb dip is satisfied in respect of:

In this first step, it is evidently possible to carry out the operationin reverse, that is to say to allow the length of the laser cavity toremain constant, therefore to set initially the value of 6 and to causea variation in the value of the steady magnetic field H, consequently inthe value of A11 in order to obtain the equality 8=Av,. In the case ofthe curve of FIG. 4, this means that the operating point of the lasercorresponds to the point K.

In a second step, an alternating axial magnetic field applied to theactive laser medium is superimposed on the steady magnetic field. Underthe action of said alternating magnetic field, the components 0* and a"of the split Doppler curve oscillate to a slight extent about theirpositions as determined by the value of the steady magnetic field H.Instead of scanning the Lamb clip by slightly modulating the value oftheoscillation frequency of the cavity laser as was the practice indevices of the prior art, the oscillation frequency of the laser cavityis allowed in this case to remain constant and the positions of theDoppler curve components are caused to oscillate to a slight extent. Inthe case of the curve of FIG, 4, the output intensity of the laser willtherefore have the same period of oscillation about the point K as thatof the alternating magnetic field. In consequence, the output intensityof the laser is modulated.

In a third step, a detector is employed in order to compare thevariations in the modulation of the output intensity with the variationsin the alternating magnetic field. Said detector delivers a correctionsignal whose intensity S increases with the magnitude of the differencebetween the variations in the alternating magnetic field and the outputintensity of the laser. The intensity S of said correction signal alsodepends on the amplitude of the alternating magnetic field H, which isapplied. In FIG. 5 which represents the intensity S of the correctionsignal as a function of the value of the alternating magnetic field Hcurve 1 indicates that the resonance condition S=Av, is fulfilled andthat the operating point of the laser is located precisely at point K ofFIG. 4 whereas, in the case of curves 2 and 3, the operating point islocated on the portions respectively to the right and to the left of thepoint K. The correction signal is then applied to means for slightlymodifying the length of the laser cavity and therefore for modifying thevalue of the parameter 8, thereby restoring if necessary the conditionsof resonance of the magnetic Lamb dip (point K of FIG. 4).

The main advantage of this method of stabilization lies in the fact thatno action is produced on the frequency to be stabilized. A furtheradvantage is the increase in intensity S ofthe correction signal whichis achieved solely by modifying the amplitude of the alternatingmagnetic field without modifying the frequency to be stabilized. Theratio ofcorrection signal to noise can thus be very high, which was notpossible in methods of the prior art which made use of the conventionalLamb dip. Moreover, since the value of the parameter Al whichcorresponds to the half-difference between two Zeeman sublevels isdirectly proportional to the value of the steady magnetic field H, theoscillation frequency of the laser which corresponds to the magneticLamb-dip resonance depends solely on the value of said steady magneticfield. In consequence, the frequency to be stabilized is adjustable.However, it is necessary to ensure that the selected value of theparameter Av, is located in a zone such that the magnetic Lamb dipremains sufficiently well defined. This zone can be evaluated at a fewhundred mc./sec. in the case of an He-Ne gas laser. By way ofindication, the width of the Doppler curve in the case of the same typeof laser is in the vicinity of 1,500 mc./sec. and the value of8 can beset, for example, at 150 mc./sec.

FIG. 6 is a diagram showing one advantageous embodiment of afrequency-stabilized single-mode gas laser which serves to carry outsaid method. The laser is constituted by an active gaseous mediumcontained in an envelope 4 and located between two mirrors 5 and 6 whichform a Fabry-Perot resonant cavity. The envelope 4 is a glass tube whichis closed at both ends by two windows placed at the Brewster angle ofincidence. The mirror 5 is a plane mirror with practically totalreflection whereas the mirror 6 is spherical and semi-relfecting. Asolenoid 7 which surrounds the envelope 4 is connected to a modulateddirect-current generator 8. The passage of said current through thesolenoid produces withinvthe interior of the envelope 4 and along theoptical axis of the laser a magnetic field which can be split into ofgauss and steady magnetic field H and an alternating magnetic field H,which are superimposed and in the same direction. The intensity of thedirect current which is delivered by the generator 8 and the amplitudeof modulation of said current are adjustable so that the density of thesteady magnetic field H and the amplitude of the alternating magneticfield H can vary. A photoelectric detector 9 detects the output light ofthe laser and is connected to one of the two input terminals of a phasedetector 10. The other input terminal of said phase detector isconnected to an output 11 of the generator 8, The detector 10 carriesout synchronous detection of variations in phase and in amplitude of themodulation of the laser output intensity with respect to the variationsin the alternating magnetic field I-l, and delivers a correction signalhaving an intensity which can increase, for example, with the differencebetween said variations. Said correction signal is amplified by means ofa direct-current amplifier l2 and is then applied to the input of apiezoelectric ceramic element 13. Said ceramic element 13 is bonded tothe rear face of the mirror 6; the vibrations of said element are thustransmitted to said mirror 6 and make it possible in a convenient mannerto modify the length of the laser cavity and therefore the resonantfrequency of said cavity. The values of the steady and alternatingmagnetic fields are respectively of the order of 70gauss and 7 gauss.Since the stability of the output frequency of the laser depends on thestability of the value of the steady magnetic field I-I, said value musttherefore be highly stable.

This invention is not limited solely to the embodimentwhich has beendescribed by way of example with reference to the accompanying drawings.In particular, the synchronous detection which is carried out by meansof the detector [0 can be carried out with the aid of other suitablemeans. The piezoelectric ceramic element 13 provides a convenient meansof modifying the length of the laser cavity to a very slight extent andtherefore of correcting the output optical frequency of the laser but itis wholly apparent that alternative devices can be employed. The steadyand alternating magnetic fields have been produced by means of asolenoid connected to a current generator 8 but it is wholly evidentthat these magnetic fields may be produced by other methods.

We claim 1. A method of frequency-stabilization of a single-mode gaslaser, wherein use is made of a correction signal resulting fromvariations in output intensity of said laser and said method consistssuccessively:

in applying a steady axial magnetic field to the gaseous active medium;in determining the value of said steady magnetic field which correspondsto the laser-cavity oscillation frequency to be stabilized and thereforeto a pre-established length of the laser cavity and conversely indetermining the length of the laser cavity which corresponds to apre-established value of said steady magnetic field so that theoscillation frequency of the laser cavity should correspond to a peakvalue of one of the two Doppler curves 0* and tr which are obtained as aresult of the Zeeman effect by splitting the Doppler curve correspondingto a zero value of the steady magnetic field or in other words insatisfying the resonance conditions of the magnetic Lamb dip;

in applying an alternating axial magnetic field which is superimposed onsaid steady magnetic field so that the split Doppler curve shouldoscillate about its position corresponding to the selected value of saidsteady magnetic field, the output light intensity of said laser beingthen modulated;

in detecting said modulated output intensity;

in comparing the variations in modulation of said output intensity withthose of the alternating magnetic field by means of a detector whichdelivers a correction signal and,

in correcting the frequency of the light emission of said laser ifnecessary by means of said correction signal and by modifying the lengthof said laser cavity.

2. A method according to claim 1 such that the optical laser emissionfrequency to be stabilized is adjusted by modifying the value of saidsteady magnetic field.

3. A frequency-stabilized laser comprising a gaseous active mediumcontained in an envelope and located between two mirrors forming aFabry-Perot resonant cavity, means for exciting said gaseous medium inorder to cause a population inversion, means for producing within saidactive medium an adjustable steady axial magnetic field and anadjustable alternating axial magnetic field, a detector for detectingthe light intensity emitted by said laser, means for comparing thevariations in said light intensity with the variations in saidalternating magnetic field, said comparison means being such as todeliver a correction signal, and means for varying the length of saidlaser cavity which are controlled by said correction signal.

4. A laser according to claim 3 said means for producing said magneticfields comprising a solenoid which surrounds said gaseous medium and amodulated direct-current generator, the intensity and modulation of saidcurrent being adjustable and the input and output terminals of saidgenerator being connected to the ends of said solenoid.

5. A laser according to claim 3 said light intensity detector being aphotoelectric detector.

6. A laser according to claim 3 said comparison means comprising a phasedetector.

7. A laser according to claim 3 said means for varying the length ofsaid laser cavity comprising a piezoelectric ceramic element which isbonded to one of said two mirrors.

1. A method of frequency-stabilization of a single-mode gas laser,wherein use is made of a correction signal resulting from variations inoutput intensity of said laser and said method consists successively: inapplying a steady axial magnetic field to the gaseous active medium; indetermining the value of said steady magnetic field which corresponds tothe laser-cavity oscillation frequency to be stabilized and therefore toa pre-established length of the laser cavity and conversely indetermining the length of the laser cavity which corresponds to apre-established value of said steady maGnetic field so that theoscillation frequency of the laser cavity should correspond to a peakvalue of one of the two Doppler curves sigma and sigma which areobtained as a result of the Zeeman effect by splitting the Doppler curvecorresponding to a zero value of the steady magnetic field or in otherwords in satisfying the resonance conditions of the magnetic ''''Lambdip''''; in applying an alternating axial magnetic field which issuperimposed on said steady magnetic field so that the split Dopplercurve should oscillate about its position corresponding to the selectedvalue of said steady magnetic field, the output light intensity of saidlaser being then modulated; in detecting said modulated outputintensity; in comparing the variations in modulation of said outputintensity with those of the alternating magnetic field by means of adetector which delivers a correction signal and, in correcting thefrequency of the light emission of said laser if necessary by means ofsaid correction signal and by modifying the length of said laser cavity.2. A method according to claim 1 such that the optical laser emissionfrequency to be stabilized is adjusted by modifying the value of saidsteady magnetic field.
 3. A frequency-stabilized laser comprising agaseous active medium contained in an envelope and located between twomirrors forming a Fabry-Perot resonant cavity, means for exciting saidgaseous medium in order to cause a population inversion, means forproducing within said active medium an adjustable steady axial magneticfield and an adjustable alternating axial magnetic field, a detector fordetecting the light intensity emitted by said laser, means for comparingthe variations in said light intensity with the variations in saidalternating magnetic field, said comparison means being such as todeliver a correction signal, and means for varying the length of saidlaser cavity which are controlled by said correction signal.
 4. A laseraccording to claim 3 said means for producing said magnetic fieldscomprising a solenoid which surrounds said gaseous medium and amodulated direct-current generator, the intensity and modulation of saidcurrent being adjustable and the input and output terminals of saidgenerator being connected to the ends of said solenoid.
 5. A laseraccording to claim 3 said light intensity detector being a photoelectricdetector.
 6. A laser according to claim 3 said comparison meanscomprising a phase detector.
 7. A laser according to claim 3 said meansfor varying the length of said laser cavity comprising a piezoelectricceramic element which is bonded to one of said two mirrors.